1. Field of the Invention
The present invention relates to a zoom lens system and an electronic image pickup apparatus using the same.
2. Description of the Related Art
In recent years, miniaturization of digital still cameras and mounting of image pickup functions on cellular phones have been advanced. In consequence, it is demanded to further miniaturize and thin image pickup lenses. As these image pickup lenses, there are demanded zoom lens systems having a zoom ratio above 2.5.
As methods for realizing the thin zoom lens systems, there are known a method of bending the optical axis in the vertical direction by disposing a reflecting member in the zoom lens system, and also a method of moving a part of lens units constituting the zoom lens system to the outside of the optical path when the image pickup apparatus is in non-use state.
However, the method of bending the optical axis by use of the reflecting member requires a space for bending rays and a space for moving the lens unit in order to secure the zoom ratio. Since these spaces are not eliminated even when the image pickup apparatus such as a camera is not used, this method is disadvantageous to decrease of the volume of the image pickup apparatus when unused. Furthermore, when the optical axis is bent, layout in the image pickup apparatus is limited.
On the other hand, in the method of moving a part of the lens units to the outside when unused, a mechanism for moving the lens units is required. Therefore, eccentricity of the lens units from the optical axis tends to occur and it is difficult to suppress the adverse influence of the eccentricity. In addition, since driving means for moving the lens units is required, it is difficult to suppress the volume of the apparatus when unused. This method is also disadvantageous in view of costs.
As another method for realizing the thin zoom lens system, a method of using a collapsible type structure in the lens barrel of the zoom lens system. As refractive power layouts of the zoom lens systems for which usual collapsible type lens barrels are intended to use, there are known a two-unit zoom lens system having a refractive power layout of a negative-positive type from an object side; a 3-unit zoom lens system of a negative-positive-negative type; and a 3-unit zoom lens system of a negative-positive-positive type.
Among them, the zoom lens system of the negative-positive type is advantageous in reducing total thickness of the lens frames which directly hold lenses since the number of the lens units is small. However, in order to decrease the total length while securing the zoom ratio, the first lens unit needs to be reciprocated in the optical axis direction and the second lens unit needs to be moved in a region which includes the equal magnification position of the second lens unit during zooming. In this case, when the second lens unit is moved to perform focusing, a movement direction of the second lens unit during focusing from infinity to a short distance is reversed in a magnification state before and after the equal magnification position of the second lens unit. Further, when the zoom lens system is focused on infinity while the second lens unit is in the equal magnification position, focusing to the short distance cannot be performed by the movement of the second lens unit. That is, the second lens unit cannot be used as a focusing lens unit. In consequence, the first lens unit or the whole zoom lens system needs to be moved for focusing, and the total length of the lens barrel including the focusing mechanism increases. Therefore, in this case, this type is disadvantageous in thinning the zoom lens system and securing the zoom ratio.
On the other hand, the zoom lens system of the negative-positive-negative type and the zoom lens system of the negative-positive-positive type are advantageous in that the increase of the total length can be suppressed by performing focusing by the third lens unit.
The 3-unit zoom lens system having a refractive power layout of the negative-positive-negative in order from the object side is advantageous to miniaturization, because it is possible to reduce the diameter of the front lens. However, since the above described type of power layout includes a negative refractive power immediately before the paraxial image surface, this type is disadvantageous in decreasing the F-number to constitute a bright zoom lens system. The incidence angle of an off-axial light flux for the maximum image height on the image surface easily increases, and the type is easily influenced by shading in a case where a CCD image sensor is used.
Moreover, although the layout is advantageous to miniaturization, the type is easily influenced by manufacturing errors, because the image is enlarged by the negative lens unit on the image side. Therefore, it is difficult to obtain a stable optical performance.
On the other hand, the 3-unit zoom lens system having a refractive power layout of negative-positive-positive in order from the object side is advantageous in that the stable optical performance can easily be obtained, focusing can be performed by the third lens unit, change of the total length of the zoom lens system during focusing is suppressed and the total length is reduced. This lens system is also preferable in respect of the performance and simplification of a mechanism.
As a zoom lens system of the negative-positive-positive type, there is known a zoom lens system in which the third lens unit moves to a position closer to the image side in the telephoto end than in the wide-angle end or hardly moves. However, in such a form of movement of the lens unit, since the third lens unit is positioned close to the image surface in the telephoto end, the height of an off-axial ray increases in the third lens unit, and the diameter of each lens easily increases. In the case where the third lens unit moves for focusing operation, when the third lens unit is positioned close to the image surface, the focusing sensitivity (the movement amount of the image surface position at a time when the focusing lens moves as much as a unit movement amount) easily decreases. Therefore, the third lens unit necessarily has a strong positive power, and it is difficult to suppress the axial thickness of the third lens unit.
As examples of the zoom lens system in which the third lens unit moves toward the object side during the zooming toward the telephoto end, there are known zoom lens systems described in Japanese Patent Application Laid-Open Nos. 2000-284177 and 2001-242378, third and fourth embodiments of Japanese Patent No. 3,513,369 and a second embodiment of Japanese Patent No. 3,606,548. The Japanese Patent Application Laid-Open Nos. 2000-284177 and 2001-242378 and Japanese Patent No. 3,513,369 disclose examples in which the second and third lens units move toward the object side during the zooming toward the telephoto end. Furthermore, Japanese Patent Application Laid-Open Nos. 2000-284177 and 2001-242378 and Japanese Patent No. 3,606,548 disclose examples in which the second and third lens units integrally move during zooming, and only the third lens unit moves during focusing.